By what measurement?
Are babies born one month prematurely all retarded? Two months? Infant brain development is a rising curve that passes through birth and continues rapidly throughout the first year. It slows, but never completely stops. The line is difficult to draw. Studies of very premature infants show that children born at 26 weeks who survive often have brain development problems, but children born later, or at full term, don't.
If a functional brain is the only criteria between "medical procedure" and "murder," the concept needs work.